COHERENT OPTICAL PHONON EXCITATION BY FEMTOSECOND LASER PULSE IN YBa₂Cu₃O_7-\delta THIN FILMS

With a correlation of nonequilibrium carriers relaxation and coherent phonons displacive excitation, the coherent optical phonon oscillations in YBa₂Cu₃O_7-\delta thin films excited by femtosecond laser pulse are simulated theoretically. It is revealed that as the oxygen concentration decreases, the coherent phonon oscillations become easier to be observed due to the decrease of the local coupling between the carriers and the lattice vibrations in the CuO₂ plane.